1. Field of the Invention
This invention involves optical annealing of damaged material.
2. Description of the Prior Art
The electrical properties of semiconductor materials are engineered to fulfill specific electrical requirements by doping the pure starting material with appropriate constituents which alter the conduction properties of the semiconductor. Such doping constituents are added to the starting semiconductor primarily by either the diffusion or the ion implantation process. These processes, especially the ion implantation process, may result in significant damage to the crystal structure of the semiconductor. Such damage renders the resultant material ineffective as an active element for most electrical applications.
In present day fabrication processes, the damage incurred during the doping step is repaired or "annealed" by bringing the material to an elevated temperature. The increased mobility and diffusion characteristics at these elevated temperatures allows both the host and the dopant constituents to reorient themselves so that a more perfect crystal structure results with concomittant electrical properties that render the devices electrically operative. Prior art annealing techniques have involved the simple use of an appropriate oven to raise the damaged material's temperature as required.
Recently, the laser has been effectively applied to this annealing process. Exposure of the damaged semiconductor to laser radiation results in increased mobility and diffusion rates necessary for effective annealing. However, unlike the prior art thermal annealing process, two specific and identifiably distinct regimes are found to occur in the laser annealing process. In the first regime, the temperature of the substrate is elevated in a manner similar to the prior art thermal process. As in the prior art process, the semiconductor retains its solid phase throughout this "solid phase epitaxial regrowth" regime.
A second laser annealing regime, without comparable precedent in the prior art annealing process, involves operating under parameters which result in the melting of that part of the substrate which is exposed to the laser. The diffusion rates and mobility in the molten phase are significantly different from that in the solid phase and, as a result, this annealing regime has radically different physical and temporal characteristics than the prior art process. When the substrate is no longer exposed to the laser energy, the molten region refreezes to a crystal, using that part of the underlying undamaged crystalline semiconductor material which is not melted as a template or seed from which to regrow. The process is consequently referred to as "liquid phase epitaxial regrowth" annealing.
The high powers which are necessary for laser annealing are currently most readily available in devices which emit radiation in the infrared region of the spectrum. However, most semiconductors are only weakly absorptive in this region of the spectrum, and hence, laser annealing with infrared sources is found to be inefficient, if at all possible, with such sources. On the other hand, while most semiconductors are found to absorb visible light effectively, currently available laser sources in this region of the spectrum have only limited power and are consequently inefficient for commercial laser annealing applications.